As a traditional method of obtaining an electrostatic developer, complex particles are added to a developer cf. Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 91143/1989. In this method, complex particles, comprising resin particles whose average grain size is smaller than that of colored particles at 0.05 to 3.0 .mu.m and inorganic microparticles bound to the surface of the resin grains, are used to keep the surface of the photoreceptor in a good condition by the polishing action thereof and hence improve the cleaning property.
However, when used for the image formation process using an amorphous silicon photoreceptor, the developer disclosed in Japanese Patent O.P.I. Publication No. 91143/1989 poses the following problems:
(1) When cleaning is performed while a cleaning device is placed on the surface of the amorphous silicon photoreceptor under relatively high pressure to improve the cleaning property in the cleaning process, the complex particles between the cleaning device and the surface of the amorphous silicon photoreceptor undergo a great pressurizing force; therefore, resin particles constituting the nuclei of the complex particles are destroyed, and the morphology and surface properties of the complex particles become poor, which results in cleaning failure.
(2) When complex resin particles are destroyed as above, resin dust occurs, which is bound to the surface of the amorphous silicon photoreceptor by the cleaning device and thus forms a film on the surface of the amorphous silicon photoreceptor. As a result, potential reduction occurs there, resulting in black dot stain and black streak stain. In addition, the surface properties of the photoreceptor deteriorate early; repeated image formation is accompanied by image density reduction.
(3) Upon complex resin particle destruction, inorganic microparticles are detached and inorganic micro dust are formed. The micro dust damages the surface of the amorphous silicon photoreceptor and fails to be cleaned off because it is embedded in the damaged portion on the surface of the amorphous silicon photoreceptor. As a result, the toner adheres to the embedded inorganic microparticles and is fixed in the image formation process which follows, leading to the problem of black dot stain and black streak strain on the image.
(4) Under high-temperature high-humidity conditions, charge leaks into the atmosphere due to too high temperature or leaks into water adsorbed to the surface of the photoreceptor, which tends to cause image failure, the phenomenon in which latent imaging failure makes the image portion corresponding to the deteriorated portion unclear or makes lateral lines lost easily. In conventional complex particles, because the inorganic microparticles are covered by hydroxyl groups, the surface adsorbs water under high-temperature high-humidity conditions; if cleaning is performed in such a state, the surface of the photoreceptor adsorbs water and leads to imaging failure upon sweeping off the residual toner.
(5) Water adsorption under high-temperature high-humidity conditions affects the triboelectric charging property of toner particles to provide a region of charge leakage. Repeated image formation leads to image density reduction and fogging.